Method and apparatus for investigating a sample particularly a pharmaceutical tablet. An emitter and/or the sample are initially positioned so that the emitter is at a predetermined distance and normal angle to a first point on a surface of the sample. The emitter then irradiates the sample with radiation having a plurality of frequencies in the range from 25GHz to 100THz at a plurality of points on the surface of the sample. Relative motion is possible between the emitter and the sample so that the surface of the sample can be tracked to maintain the predetermined distance and normal angle at each of the plurality of points, and allow radiation transmitted and/or reflected from the sample at the plurality of points to be detected. This has particular application to imaging the structure or composition of a coating on a pharmaceutical tablet.